The present invention relates generally to data or voice communication, and more specifically to synchronization in a communication system.
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (N) orthogonal frequency subbands. These subbands are also referred to as tones, sub-carriers, bins, and frequency channels. With OFDM, each subband is associated with a respective sub-carrier that may be modulated with data, pilots, or overhead information.
In an OFDM system, a transmitter processes data to obtain modulation symbols, and further performs processing on the modulation symbols to generate an OFDM symbol. The transmitter then conditions and transmits the OFDM symbol via a communication channel. The OFDM system may use a transmission structure whereby data is transmitted in frames, with each frame having a time duration. Different types of data (e.g., traffic/packet data, overhead/control data, pilot, and so on) may be sent in different parts of each frame. The term “pilot” generically refers to data and/or transmission that are known in advance by both the transmitter and a receiver.
The receiver typically needs to obtain accurate frame and OFDM symbol timing in order to properly recover the data sent by the transmitter. For example, the receiver may need to know the start of each frame in order to properly recover the different types of data sent in the frame. The receiver often does not know the time at which each OFDM symbol is sent by the transmitter nor the propagation delay introduced by the communication channel. The receiver would then need to ascertain the timing of each OFDM symbol received via the communication channel in order to properly perform the complementary OFDM demodulation on the received OFDM symbol.
Synchronization refers to a process performed by the receiver to obtain frame and OFDM symbol timing. The receiver may also perform other tasks, such as frequency error estimation and channel estimation. Synchronization can occur at different times to improve timing and correct for changes in the channel. Wireless systems can have their timing change coherently since abrupt changes in the channel are unlikely.
Often the channel experiences varying delay and multi-path. Different reflections or paths of the signal can reach a receiver at different times and have different magnitudes. Fading affects the magnitude of the received signal. A delay spread is the difference between the first arriving path (FAP) and a last arriving path (LAP). The LAP may not be the last actual reflection that is received, but the last one that meets some time delay limit and/or magnitude criteria. If both FAP and LAP can be estimated correctly and the OFDM symbol timing is adjusted accordingly, then most of the received signal reflections can be constructively used for data demodulation.